1. Field of the Invention
This invention pertains to an IR focal plane array useful in IR imaging application, particularly to arrays formed by spin casting a plastic film onto a wafer, and more particularly to such a sensor where a pyroelectric film with top and bottom electrodes is deposited directly on a semiconductor wafer containing the readout structure.
2. Description of the Prior Art
Accurate and reliable temperature measurements and imaging are performed using solid-state lasers and non-linear optical materials, laser transmitter, detectors and LIDAR subsystems, and infrared detectors.
Photon detectors based on technologies such as lead salts, Schottky barriers and indium antimonide are also currently available. However, these detectors require cryogenic cooling to achieve high sensitivities. Furthermore, the quantum nature of the photon absorption implies not only a spectral cutoff wavelength but also a sensitivity which varies with respect to wavelength.
IR thermal detectors based on thermistor bolometers, thermopiles and pyroelectric detectors have been available for several decades. However, these suffer from the drawback of lower speeds in comparison with photon detectors.
To date, infrared imagery containing focal plane arrays of more than 80,000 "uncooled" detectors sensitive to infrared radiation in the 8 to 14 micron wavelength region have been fabricated and are available commercially. These detectors do not require cryogenic cooling or mechanical scanning, and have demonstrated noise-equivalent temperature difference (NETD) values of 0.1.degree. C. Two different detector technologies, one ferroelectric and the other bolometric, have been used in these focal plane arrays. The uncooled sensor technology has been incorporated into commercial devices such as security sensors, weapon sights, handheld surveillance devices and for "night vision" displays.
However, these detectors have been found to be unsatisfactory. Uncooled focal plane array technologies, based on ferroelectronics, suffer from the requirement of bump-bonding to a readout circuit, typically active silicon electronic circuitry. Bump-bonding is a technique where the readout electronics of the m * n array and the detector array are two separate entities which are bonded together physically by an interfacing material, typically indium, which is in the shape of bumps or mounds on each of the m * n detector pixels and corresponding readout electronics (see FIG. 5). The detector array and the readout chip are physically aligned to form a one-to-one correspondence between each of the m * n bumps. The bump bonds are formed by heat or pressure until the detector array and readout chips are fused together. The bump-bonding approach is further plagued by quality-control problems such as mechanical damage to the detector due to the cold weld process, alignment and surface oxidation present on the "bump" material. Reticulated lithium tantalate pyroelectric arrays have also been used to fabricate uncooled focal plane arrays but also suffer from the requirement of bump-bonding.
Moreover, the detectors used in bump technology require reticulation due to the detector element thickness (typically 30 microns) of the pixel elements in order to reduce the thermal crosstalk, which is otherwise quite significant and causes deterioration of the modulation transfer function (MTF).
The present invention overcomes the manufacturing difficulties associated with bump bonding of pyroelectric detector arrays by utilizing an integral process where the detector array is grown directly on the wafer containing interface and readout electronics.